1. Field of the Invention
The invention relates to optical communication systems and processes.
2. Discussion of the Related Art
In optical communications systems, transmission optical fibers cause chromatic dispersion and nonlinear optical effects. Both effects degrade optical pulses in ways that increase transmission errors. To reduce the pulse degradation caused by chromatic dispersion, optical communication systems typically incorporate dispersion compensation devices, e.g., dispersion compensating fibers (DCFs), dispersion compensating interferometers, or dispersion compensating grating systems, to compensate for chromatic dispersion. To reduce the pulse degradation caused by nonlinear optical effects, long-haul optical communication systems typically manage dispersion through special processes and special dispersion maps.
The special processes involve transmitting optical pulses over transmission spans in a pseudo-linear transmission regime (PLTR). The PLTR is defined by the following operating conditions: a bit rate of 10 Giga bits per second (Gb/s) or higher, a wavelength of 1.25 micrometers (μm) to 1.7 μm, a pulse full width at half maximum power of 60 ps or less, and a pulse duty cycle of between 10% and 70%. The transmission spans are typically single mode optical fibers with high magnitude of the dispersions that are about +2 pico seconds (ps) or more per nanometer (nm) per kilometer (km) at communication wavelengths. Due to this high dispersion, the transmission spans cause substantial broadening of optical pulses. In the PLTR, this pulse broadening causes multiple optical pulses from nearby frequency channels to substantially overlap in time, which averages inter-pulse interactions and reduces pulse distortion from inter-channel interactions.
The special dispersion maps result from chromatic dispersion compensation devices that are located at the input and/or output ends of transmission spans. Herein, a dispersion map plots the cumulative dispersion as a function of transmission distance along an optical communication path. The dispersion compensation devices at the input and/or output ends of transmission spans produce abrupt changes in cumulative dispersion along the communication path thereby producing nontrivial dispersion maps.
FIG. 1 shows a dispersion map 8 of an optical communication path with eight identical transmission spans. Each transmission span includes 100 km of positive dispersion single mode fiber (SMF) 10. Between the ends of the SMFs 10 of adjacent transmission spans is a DCF 12. Along each transmission SMF 10, the cumulative dispersion increases linearly with distance. Along each DCF 12, the cumulative dispersion decreases linearly with distance. The lengths of the DCFs 12 are selected to produce full compensation of the dispersion 14 that accumulated on the preceding transmission SMF 10. This full compensation of chromatic dispersion produces a dispersion map in which the cumulative dispersion is periodic in a span-by-span manner. This dispersion map is referred to as a full-span compensation map.
While full-span compensation maps do compensate very well for the pulse degradation caused by chromatic dispersion, these maps do not completely correct for the pulse degradation caused by nonlinear optical effects. In particular, the span-by-span periodicity suggests that any residual pulse degradation from nonlinear optical effects will add constructively with the number of spans as a pulse travels along the optical communication path. In long haul communication systems, this constructive accumulation of nonlinear distortions can be the primary contribution to the bit error rate (BER) and pulse degradation:
FIG. 2 shows a dispersion map 16 for an optical communication system that includes a series of identical transmission spans of positive dispersion SMF, a DCF located after each transmission span, a pre-transmission dispersion compensator, and a post-transmission dispersion compensator. The dispersion map 16 provides pre-transmission dispersion compensation, CPRE, prior to the first span, in-line dispersion compensation, CIL, after each transmission span, and post-transmission dispersion compensation, CPOST, after the last transmission span. In the dispersion map 16, the in-line dispersion compensation, CIL, does not entirely compensate for the positive dispersion that accumulates in the preceding transmission span of positive dispersion SMF. Instead, a residual dispersion per span, CRDPS, remains after the in-line dispersion compensation for each span. A nonzero and constant value of CRDPS, as e.g., in the map 16, produces a dispersion map that is referred to as a singly periodic map. If the value of CRDPS is the same and zero for each span as, e.g., in FIG. 1, the dispersion map is referred to as a full-span compensation map.
Some special dispersion maps better compensate for the pulse degradation caused by nonlinear optical effects. In particular, it is believed that optimal values of CPRE exist for singly periodic dispersion maps that are EDFA pumped. For such maps, the approximately optimal CPRE is believed to satisfy:CPRE=−NCRDPS/2+(D/α)ln([1−exp(−αLspan)]/2)Here, α is the power loss per unit length in a transmission span, N is the total number of spans, D is the dispersion in optical fibers of the transmission spans, and Lspan is the length of each span. The above equation defines CPRE in terms of CRDPS when the physical parameters of the transmission spans, i.e., Lspan, D, and α, are given. A singly periodic map that satisfies the above optimization equation will compensate well for the effects of intra-channel cross-phase modulation and intra-channel four-wave mixing.
Among dispersion maps that satisfy the above optimization equation, singly periodic maps with small CRDPs's seem to produce large amounts of timing jitter in transmitted optical pulses. Normally, large amounts of timing jitter are not desirable, because timing jitter can cause reception errors.
Various nontrivial dispersion maps are described in U.S. Pat. No. 6,583,907 issued Jun. 24, 2003; U.S. Pat. No. 6,606,176 issued Aug. 12, 1999; and U.S. patent application Ser. No. 10/152,645, filed May 21, 2002 by R.-J. Essiambre et al all of which are incorporated herein by reference in their entirety. While special dispersion maps have reduced the amount of pulse degradation from nonlinear optical effects, processes for further reducing such pulse degradation are desirable. Such processes could enable higher bit rates and/or higher power levels that allow optical transmission of data over longer distances.